Process for production of ammonia and derivatives, in particular urea

ABSTRACT

A process for producing ammonia and a derivative of ammonia from a natural gas feed comprising conversion of natural gas into a make-up synthesis gas; synthesis of ammonia; use of said ammonia to produce said derivative of ammonia, wherein a portion of the natural gas feed is used to fuel a gas engine; power produced by said gas engine; is transferred to at least one power user of the process, such as a compressor; heat is re-covered from exhaust gas of said gas engine;, and at least part of said heat may be recovered as low-grade heat available at a temperature not greater than 200° C., to provide process heating to at least one thermal user of the process, such as CO 2  removal unit or absorption chiller; a corresponding plant and method of modernization are also disclosed.

FIELD OF APPLICATION

The invention concerns a method for producing ammonia and derivatives ofammonia, particularly urea, starting from natural gas. The inventionalso discloses a method of modernizing an ammonia-urea plant.

PRIOR ART

The integrated production of ammonia and derivatives of ammonia is knownin the fertilizer industry. For example the production of ammonia andurea is known.

Ammonia production, usually from a natural gas feed, involves conversionthe natural gas into a synthesis gas in a front-end section and theconversion of said synthesis gas into ammonia in a synthesis loop. Theconversion of the natural gas feed into synthesis gas usually involvestwo-stage steam reforming, although autothermal reforming and partialoxidation are feasible options, followed by purification including shiftconversion of CO to CO2, removal of CO2 and optionally methanation. Theresulting purified gas is termed ammonia synthesis gas. A process forproducing ammonia synthesis gas is described for example in EP 2 065337.

In a so-called ammonia-urea plant, at least part of the synthesizedammonia is reacted with carbon dioxide to produce urea.

A plant for the production of ammonia and its derivatives comprises alsoa complex steam system including both steam producers and steam users.The steam producers recover process heat from various process streams,mostly from conversion of the natural gas feed into generation of rawsynthesis gas (usually by steam reforming) and from its subsequentpurification. The steam users include for example one or more steamturbines to drive equipment such as compressors.

The main steam users of an ammonia-urea plant are the driving turbinesof large gas compressors such as the synthesis gas compressor whichraises the pressure of the generated make-up gas to the pressure of thesynthesis loop, and other compressors for process air, ammonia, carbondioxide or natural gas.

The steam system uses typically a Hirn cycle (or superheated steamRankine cycle) to produce power. Said cycle as applied in ammonia plantshowever is relatively inefficient, with less than 30% efficiency andtypically only 26-27% even for relatively modern and large plants.Hence, less than 30% of the heat used to raise/superheat the steam isconverted into mechanical power, with more than 70% lost mainly toair/water cooling in the steam condenser and in other inefficiencies.

A part of the steam produced in the steam system is also used in thereforming process. This fraction of steam is called process steam. Arelevant parameter of the reforming process is the steam-to-carbon ratiowhich is the molar ratio between steam and carbon introduced in theprocess. Said ratio is normally around 3.

The steam generated by process heat recovery is generally not enough tocover all requirements, and the shortfall is covered in the prior art byinclusion of a gas-fired auxiliary boiler to generate the additionalsteam required. Said boiler introduces however an additional consumptionof natural gas, increasing the overall consumption for a given capacityin terms of ammonia which is synthesized. Said additional consumption isa drawback especially in a location where the natural gas is expensiveand/or is available in a limited amount.

Existing and new plants in these areas of high natural gas cost mustminimize the consumption of natural gas to be competitive on the globalfertilizer market. Moreover, where the total natural gas available forfertilizer production is limited, any reduction in the specific gasconsumption of the plant enables a corresponding increase of thefertilizer production capacity.

The main natural gas consumer of a fertilizer complex for the productionof a derivative of ammonia, such as urea, is the ammonia plant. Althoughmany efforts have been made to reduce the energy consumption of ammoniaplants, the processes available today are not efficient enough, or aretoo expensive to operate, where gas costs are high. Even more so, theexisting methods for revamping existing ammonia plants are notcompetitive as they generally address the requirement to increase thecapacity of existing units rather than minimizing the gas consumption.

In recent times, limitations of quantity of natural gas available forthe ammonia plants have emerged. Such limited availability may comprisea physical gas shortage due to the reduction of the production of thegas fields, or may be due to commercial and/or economic reasons likerunning out of the contractual share of gas available to the plantand/or a different scenario of price and demand for natural gas.

In addition to the above-mentioned large compressors, a conventionalammonia or ammonia-urea plant typically contains several smallerrotating machines, such as small compressors, fans and pumps.Historically these smaller machines have been directly driven by steamturbines having a low efficiency. In a few existing ammonia plants, somesmall steam turbines have been replaced with electric motors in order toreduce operating costs, while in some new plant designs most or all ofthe smaller machines are electrically driven, with the necessary powerprovided by a central generator driven by a more efficient steamturbine. Even in the case of those new plants designs, however, theoverall fuel-to-power thermal efficiency for driving the smallermachines is less than 30%.

Hence there is still the need to increase the fuel-to-power efficiencyof the driving system of the various machines (including the smallercompressors, fan and pumps) of ammonia plants.

SUMMARY OF THE INVENTION

The problem faced by the invention is to solve the above drawbacks andreduce the energy consumption of the ammonia plants based on natural gasfeed. In particular, a purpose of the invention is to reduce the amountof natural gas which is consumed in gas-fired boilers to generate steamrequired by the power users of the plant.

The idea underlying the invention is to furnish at least part of thepower demanded by the smaller rotating machines, such as small gascompressors, fans, pumps by means of reciprocating gas engines.

Accordingly, the above problem is solved by a process for producingammonia and a derivative of ammonia from a natural gas feed, accordingto claim 1.

The term of reciprocating gas engine, in this description, denotes areciprocating internal combustion engine comprising one or morereciprocating pistons inside respective cylinders. Said gas engineoperates on 2-stroke or 4-stroke cycle, and is fuelled with a gaseousfuel such as for example natural gas.

Reciprocating gas engines are available with fuel-to-power thermalefficiency of 45-50%. Then, the provision of power to the said smallerrotating machines by said gas engine(s) in substitution for steamturbines will enable a reduction in the quantity of steam generated inthe above-mentioned auxiliary steam boiler. As the fuel-to-power thermalefficiency of the gas engine(s) is higher than for the correspondingsteam cycle, there will result a significant saving in the natural gasconsumption of the whole ammonia or ammonia-urea plant.

Power produced by the gas engine can be transferred to the users by anelectric motor using electricity produced by the gas engine, whilemechanical coupling, (for example by means of a fluid coupling) may alsobe used. At least some of the heat contained in the exhaust of said gasengine is recovered for use in the thermal users of the plant. Heatcontained in the coolant of the gas engine may also be used in the plantas a low-grade heat.

The term of low-grade heat denotes heat which is made available to thethermal users at a temperature of 200° C. or less. According to theinvention, said low grade heat is recovered by means of a suitable heatexchange medium (also termed heat sink fluid) which is indirectly heatedby the exhaust gas of the gas engine to a temperature which is notgreater than 200° C. Said heat exchange medium may be, for example,water which is heated or partially or completely evaporated.

A preferred feature of the invention is a regulation of the globalsteam-to-carbon ratio (SC ratio) of the front-end section to a valuelower than conventional. Preferably said SC ratio is regulated to avalue of less than 2.7 and more preferably in the range 2.3 to 2.6.Although lowering the SC ratio has certain advantages for the ammoniaproduction process as outlined below, it has the disadvantage ofdetermining a reduction in the quantity of steam available from heatrecovery for power generation; this shortfall is however compensated inthe present invention by the provision of said gas engine. Hence, theadvantages of a lower SC can be fully exploited.

Beneficial effects of a low SC ratio include: the steam required for thereforming process is reduced; the heat duty of the steam reformer forpreheating and reforming the mixed feed is reduced for a given ammoniaproduction, and so is the reformer fuel consumption; less heat isrejected to cooling water / air at the end of the heat recovery train inthe front end, after recovering the valuable heat from the syngas (steamis in fact added in large excess of steam the requirements for thereforming and CO shift, and the excess steam is substantially allcondensed before the synthesis); and the mass flow rate in the front endis lower.

Preferred ways of facilitating a reduction in the SC ratio includeprovision of a pre-reformer and use of improved catalysts in the steamreformer and CO shift reaction stages.

According to the invention, heat, particularly low-grade heat, isrecovered from exhaust of said gas engine. Said heat can be used for anumber of purposes including, but not limited to, the followingexamples: heating of a heat transfer medium such hot water or thermaloil, the regeneration of a CP2-rich solution in a CO2 removal unit, theheating of the reboiler of an absorption refrigeration chiller, thedistillation of an ammonia-rich aqueous ammonia solution, and/or theinitial preheating of natural gas, process air, combustion air.

According to different embodiments, said low-grade heat as defined aboveconstitutes only a portion of the total heat that can be recovered fromthe exhaust of the gas engine. Heat recovered from the gas engineexhaust at a higher temperature, for example an exhaust temperaturegreater than around 250° C., may be used for example for the generationor superheating of steam suitable to drive a turbine. One embodiment ofthe invention provides that: a first portion of heat recovered fromexhaust of said gas engine is used in a heat recovery steam generator toproduce superheated steam and said steam is expanded in a backpressureor extraction steam turbine producing further mechanical power, thusforming a combined cycle, and a second portion of heat recovered fromexhaust of said gas engine is used to provide said low-grade heat. Morepreferably, a steam flow taken from said backpressure or extractionsteam turbine can be further used to provide heating to at least one ofsaid thermal users.

Part of the steam produced can also be exported if an external user isavailable.

According to some embodiments of the invention, part of the fuel for thegas engine is a waste fuel stream. For example a waste fuel stream forthe gas engine may include: a purge gas taken from said ammoniasynthesis loop, or a tail gas from a loop purge recovery unit. Said unitfor example recovers hydrogen from a purge gas which is withdrawn fromthe ammonia synthesis loop. This is another advantage and synergisticeffect of the invention, because the purge gas from the ammoniasynthesis loop can be used as fuel in the gas engine, even if at lowpressure.

The heat recovered from exhaust gas may provide direct or indirectheating to the thermal users, according to the various embodiments ofthe invention. Indirect heating may comprise for example the heating ofa suitable heat transfer medium or the pre-heating of selected streamssuch as pre-heating of fuel or pre-heating of combustion air of a firedreformer.

An exemplary embodiment of direct heating is direct use of exhaust gasfrom said gas engine as combustion medium (gas stream containingoxygen), in a fired reformer or other fired heater. Accordingly, theheat of the exhaust gas is directly transferred to the combustionprocess, in a manner which is energetically equivalent to pre-heating offuel or of combustion air.

The invention is synergistic in particular with carbon dioxide removaltechniques which require a low-grade heat. For example, removal ofcarbon dioxide by chemical or chemical-physical absorption needs a heatinput used for regeneration of a CO2 removal solution. Said solution mayinclude amine or potassium carbonates or similar.

The above mentioned derivative of ammonia may be, for example, one ormore of urea, phosphates or nitric acid. Preferably said derivative isurea. A preferred application of the invention relates to ammonia-ureaprocesses and plants, where some or all of the synthesized ammonia isreacted with carbon dioxide to produce urea.

Another aspect of the invention is a method of modernizing a plant forproducing ammonia and a derivative of ammonia, particularly urea,according to the attached claims.

Said method is characterized by the provision of: at least onereciprocating gas engine; suitable power transfer means to transfer thepower produced by said engine to at least one of the power users; heatrecovery means for recovering heat, particularly low-grade heat, fromthe exhaust gas of said gas engine, and also comprises the provision ofsaid low-grade heat to at least one of the thermal users of the plant,or to at least one newly-installed thermal user.

In some embodiments, the modernization comprises the installation of newthermal users. A newly-installed thermal user, in some embodiments, mayreplace an existing power user. For example, the ammonia sectionnormally comprises a vapour compression refrigerator for condensation ofthe produced ammonia, and the invention may comprise the replacement ofsaid cycle with an absorption refrigerator which uses low-grade heatinstead of mechanical power.

Hence, an aspect of the invention is to provide a suitable low-gradeheat sink, to exploit the heat recovered from the gas engine exhaust.This may be done by lowering the steam-to-carbon ratio, as stated above,and/or by installing new thermal users.

An advantage of the invention is that the gas engine alone can reach athermal efficiency of more than 45% on LHV (low heat value of fuel)basis, and an efficiency of over 50% can be achieved when a heatrecovery steam generator (HRSG) and associated back-pressure orextraction steam turbine, as described above, are also provided. Theseefficiency values are considerably higher than the typical efficiency ofthe steam cycle in an ammonia plant, resulting in a reduction ofconsumption of natural gas fuel and hence in the total gas consumptionof the ammonia plant.

Another advantage is the strong integration and unexpected synergisticeffect between the provision of said power unit, the lowering of the SCratio, and the feeding of low-grade heat to the existing ornewly-installed thermal users.

The invention is particularly advantageous in conjunction with chemicalor chemical-physical absorption technique for the removal of carbondioxide. A CO2 removal unit with chemical or chemical-physicalabsorption is a major user of low-grade steam; the remaining steam afterabstraction of steam needed for CO2 solvent regeneration can beexported, but the amount exportable is generally limited. Reducing theSC ratio has the effect that less steam (i.e. less amount of low gradeheat) is available, which would be perceived as a drawback in the priorart. This drawback could be theoretically solved by implementation of aphysical absorption CO2 removal unit, which would require less heat forsolvent regeneration than a chemical or chemical-physical CO2 removalunit, but this would entail a significant capital cost. The inventionovercomes this problem, thanks to the possibility to recover CO2 removalsolvent regeneration heat from exhaust gas of the gas engine.

The invention will be further elucidated by the following description ofan embodiment thereof, given by way of non-limiting example withreference to the attached FIG. 1.

DETAILED DESCRIPTION

FIG. 1 illustrates a scheme of a process for ammonia synthesis fromnatural gas, according to a preferred embodiment of the invention.

Block 1 denotes an ammonia-urea plant comprising: an ammonia synthesissection, comprising a front end section and a high pressure synthesisloop, and a urea plant where some or all of the ammonia is reacted withcarbon dioxide to produce urea.

Said front end section comprises preferably a steam reforming sectionand a purification section. Said steam reforming section comprises forexample a primary steam reformer and a secondary reformer. Saidpurification section may include shift converters of CO to CO2, a CO2removal unit and, optionally, a methanator.

The ammonia-urea plant 1 comprises a number of power users 2 and thermalusers 3. Said power users (PU) include gas compressors, fans and pumps.Thermal users (TU) typically use steam as a source of heat and includefor example the CO2 removal unit where heat is needed for regenerationof a CO2 removal solution.

A portion 15 of the available natural gas feed NG is used to fire areciprocating gas engine 6, including a plurality of cylinder-pistonassemblies.

The power produced by said gas engine 6 is transferred to one or more ofthe PUs (line 7) in electrical or mechanical form, that is viaconversion into electrical energy or direct mechanical coupling.

For example, in a first embodiment a PU such as a pump is powered by anelectric motor powered at least in part by electrical energy produced bya generator driven by said gas engine 6; in a second embodiment said PUis mechanically coupled to said gas engine.

The power produced by gas engine(s) 6 hence will replace one or more ofthe steam turbines of the prior art.

Exhaust gas flow 8 discharged by said gas engine 6 is fed to a heatrecovery unit 9. Said recovery unit 9 produces a low-grade steam 10 byevaporating a feed water 14. Said steam 10 has a temperature not greaterthan 200° C., preferably in the range 150-200° C., and is used in atleast one of the TUs 3 of the ammonia section 1. The cooled exhaust gasleaves the recovery unit 9 at line flow 11.

A particularly preferred use for low-grade steam 10 is regeneration ofCO2 removal solution in the CO2 removal unit of the purificationsection. Removal of carbon dioxide is preferably carried out with any ofthe following methods: amines, or activated amines, or potassiumcarbonate.

Since the gas engine exhaust gas 8 is usually at a higher temperature(e.g. 350-400° C.), the heat recovery unit may also provide anadditional amount of mechanical or electrical power, as indicated byline 13, for example via a heat recovery steam generator (HRSG) and abackpressure or extraction steam turbine.

In a preferred embodiment, the global steam-to-carbon ratio in thefront-end section of the plant 1 is regulated at a low value of lessthan 2.7, preferably in the range 2-2.6 and more preferably in the range2.3-2.6. As stated above, the reduction of said ratio has a positive andsynergistic effect with the provision of the gas engine(s) 6 and of theheat recovery unit 9.

The global steam-to-carbon ratio can be reduced in conjunction with oneor more of the following: by installing a pre-reformer upstream theprimary reformer; bypassing a portion of natural gas (typically morethan 10% of the reformer feed) around the steam reformer tubes andsending it directly to the secondary reformer.

In some embodiments, the ammonia-urea plant 1 comprises a hydrogenrecovery unit (HRU). The tail gas 12 of said HRU may be used as fuel inthe gas engine(s) 6 as shown in FIG. 1. For a revamp, this is veryconvenient compared to the recycle in the steam reformer, because itavoids the otherwise typically necessary modification of the steamreformer burners. This is possible even if the tail gas of the HRU is atlow pressure, such as for instance from a cryogenic HRU or a PSA.

Additional steam 4 for the thermal users 3 can be optionally provided bya gas-fired auxiliary boiler 5.

Further preferred aspects of the invention are the following. Energy canbe saved by installing a means for recovering reactants (H2 and N2) fromthe synthesis loop purge, while effectively rejecting the inerts (Ar andespecially CH4). Such means may include a membrane, or on adsorbents, orpreferably a cryogenic HRU which recovers most of the reactants at apressure preferably of at least 60 bar and preferably more than 100 bar.

Both reducing the S/C ratio alone and installing a purge gas recoveryHRU alone provides some energy saving, but there is synergy in applyingboth solutions together.

In fact, a lower S/C ratio reduces the methane conversion in thereforming process, increasing the residual methane concentration in themake-up gas and ultimately in the synthesis loop. This offsets saving inprocess steam consumption. However, coupling an HRU with a lower S/Cratio eliminates the drawbacks of the latter, i.e. the increased methaneconcentration in the synthesis loop, while retaining the benefits ofboth: reduced firing, less inerts in the synthesis loop, H2 and N2recovered at high pressure.

Depending on the selected S/C ratio, either a high temperature (HTS) ora medium temperature (MTS) shift may be deployed. A HTS allows recoverof a higher level heat, hence ensuring a higher overall efficiency andless gas consumption. However, HTS can be used only down to a global S/Cratio of about 2.6-2.7. In some cases it may be useful to reduce the S/Cratio to lower values, hence MTS is then required. The MTS can beadiabatic or isothermal. Isothermal MTS means that the shift convertercontains a heat exchanger adapted to keep the temperature of the shiftconverter product gas within a desired range. Adiabatic MTS can be usedwhen the amount of heat released in the shift converter is limited, forexample when the oxidant in the secondary reformer is air and theconcentration of CO inlet to the shift is not too high.

1. A process for producing ammonia and a derivative of ammonia from anatural gas feed (NG) comprising: conversion of natural gas intosynthesis gas in a front-end section; synthesis of ammonia from saidsynthesis gas in a synthesis loop; use of at least part of said ammoniato produce said derivative of ammonia, said process being carried outwith power users requiring a mechanical power for operation, and thermalusers requiring a heat input for operation; wherein: a portion of saidnatural gas feed is used to fuel a reciprocating gas engine; powerproduced by said gas engine is used to cover at least partially thepower demand of said power users; heat is recovered from exhaust gas ofsaid gas engine, and at least part of said heat is recovered, to provideheating to at least one of said thermal users, wherein heat recoveryfrom said exhaust of said gas engine is at least in part low grade heattransferred to at least one of said thermal users via a heat exchangemedium, said medium being heated by indirect heat exchange with theexhaust gas to a temperature which is not greater than 200° C.
 2. Theprocess according to claim 1, said power being transferred from said gasengine to at least one of said power users in an electrical ormechanical form.
 3. The process according to claim 1, wherein saidconversion of natural gas into synthesis gas in said front-end sectionis carried out by steam reforming with a global steam-to-carbon molarratio not greater than 2.7.
 4. The process according to claim 1, whereinsaid heat recovered from said gas engine is used to provide heat to oneor more of the following thermal users, by means of at least one of thefollowing: heating of a heat transfer medium such hot water or hot oil,the regeneration of a CO2-rich solution in a CO2 removal unit, thepowering of an absorption refrigeration chiller, the distillation of anammonia-rich aqueous ammonia solution, heating of natural gas or otherfuel, heating of process air, heating of combustion air, direct use ofgas engine exhaust as a combustion medium.
 5. The process according toany of the previous claim 1, wherein: a first portion of heat recoveredfrom exhaust of said gas engine is used in a heat recovery steamgenerator to produce steam and said steam is expanded in a backpressureor extraction steam turbine producing further mechanical power, thusforming a combined cycle, and a second portion of heat recovered fromexhaust of said gas engine is used to provide said low-grade heat. 6.The process according to claim 5, wherein a steam flow taken from saidbackpressure or extraction steam turbine is used to provide heating toat least one of said thermal users.
 7. The process according to claim 1,wherein said conversion of natural gas into synthesis gas comprises aprimary steam reformer and a secondary reformer, or a pure autothermalreformer, or a partial oxidation reactor, obtaining a raw synthesis gas,and a purification of said raw synthesis gas, comprising at least ashift reaction and removal of carbon dioxide from the shifted gas. 8.The process according to claim 7, said shift conversion being a hightemperature shift (FITS) on iron-based catalyst, or a medium temperatureshift (MTS) on copper-based catalyst.
 9. The process according to claim7, said removal of carbon dioxide being carried out with any of thefollowing methods: amines, or activated amines, or potassium carbonates.10. The process according to claim 1, said derivative of ammonia beingurea.
 11. The process according to claim 1, in which said power usersare at least one of a compressor, a fan or a pump.
 12. The processaccording to claim 1, wherein said exhaust gas from said gas engine isused to supply in part the combustion medium for a fired reformer orother fired heater.
 13. The process according to claim 1, wherein partof the fuel for the gas engine is a waste fuel stream such as: a purgegas taken from said ammonia synthesis loop, or a tail gas from a looppurge recovery unit.
 14. A plant for producing ammonia and a derivativeof ammonia, particularly urea, from a natural gas feed (NG), said plantcomprising: a front-end section for generation of ammonia make-upsynthesis gas; a synthesis loop for synthesis of ammonia from saidmake-up synthesis gas; a section for the conversion of at least part ofthe synthesized ammonia into said derivative; power users requiring amechanical power for operation, and at least one thermal user requiringa heat input for operation; wherein said plant further comprises: atleast one reciprocating gas engine, the power produced by said gasengine being transferred to at least one of said power users, heatrecovery means for recovering heat from exhaust of said gas engine, andin that said plant further comprises: heat recovery means for recoveryof low-grade heat from exhaust of said gas engine via a heat exchangemedium, said heat recovery means comprising indirect heat exchange meansarranged to heat said medium to a temperature not greater than 200° C.,and said plant comprises means arranged to transfer said low-grade heatto at least one of said thermal users.
 15. A method of modernizing aplant for producing ammonia and a derivative of ammonia, particularlyurea, wherein: said plant comprises a front-end section for generationof ammonia make-up synthesis gas; a synthesis loop for synthesis ofammonia from said make-up synthesis gas; a section for the conversion ofat least part of the synthesized ammonia into said derivative; the plantalso comprising power users and thermal users; wherein: the provision ofat least one reciprocating gas engine, and the provision of suitablepower transfer means to transfer the power produced by said engine to atleast one of said power users, the provision of heat recovery means forrecovering a heat from exhaust gas of said gas engine, and the provisionof heat recovery means for recovering a low-grade heat from exhaust gasof said gas engine, by indirect heat exchange with a medium, said mediumbeing heated by the exhaust gas to a temperature not greater than 200°C., the provision of the so recovered low-grade heat to at least one ofsaid thermal users of the plant, or to at least one newly-installedthermal user.
 16. The method according to claim 15, wherein theprovision of said power transfer means includes: the provision of anelectrical motor and the provision of an electrical generator coupled tosaid gas engine.
 17. The method according to claim 16, saidnewly-installed thermal user being one of the following: a reboiler of aCO2-rich solution in a CO2 removal unit, a reboiler of an absorptionrefrigeration chiller, a reboiler of an ammonia-rich aqueous ammoniasolution distillation system, a preheater of a natural gas or fuel gas,a preheater of process air, a preheater of combustion air.
 18. Themethod according to claim 15, further comprising the step of reducingthe global steam-to-carbon ratio of the front-end section to a valuelower than the original.
 19. The process according to claim 1, whereinsaid conversion of natural gas into synthesis gas in said front-endsection is carried out by steam reforming with a global steam-to-carbonmolar ratio in the range 2.3 to 2.6.
 20. The method according to claim18, wherein said global steam-to-carbon ratio of the front-end sectionis in a range of 2.3 to 2.6.